1. Field of the Invention
This invention relates to a so-called porosimeter, that is a measuring apparatus capable of recording volume and size of superficial microcavities, or pores, in solid substances, establishing a biunivocal correspondence, at least per points, between a determined average radius dimension of pores and their volume. In other words, the apparatus is capable of determining the volume of the solid pores having a certain average radius and, subsequently, the volume of the pores having ever lower average radius.
2. Description of the Prior Art
Similar apparatuses are already known and operate according to the so-called Drake method, which essentially consists in placing the solid under test into a vessel, creating vacuum in this vessel, filling same with mercury and submitting mercury to ever increasing pressures. For each specific pressure valve it is possible to establish an average radius of the pores into which mercury penetrates. In other words, each pressure value corresponds to a value of the pores average radius, above which pores are filled with mercury, while mercury, on the contrary, does not penetrate into pores having lower average radius.
This average radius value is given by the following relation: EQU Rm=(2.sigma.cos .theta.).alpha./p
where:
Rm=average radius of pores, in A; PA1 .sigma.=mercury vapour tension at 25.degree. C. (temperature at which analysis is carried out); PA1 .theta.=contact angle between mercury and material under test. This angle is known for any material and has a value of approx. 140.degree., anyway always ranging between 130.degree. and 150.degree.; PA1 p=pressure exerted.
It is therefore obvious that, knowing .sigma. and .theta. values and measuring p value, it is easy and automatic to correlate p to Rm and obtain the latter value from easy knowledge of p, exactly measurable by means of any suitable known system. In order to obtain the required biunivocal correlation between the pores volume and the relevant average radius Rm, it is then necessary to perform, for each Rm value, that is for each p value, an exact measurement of mercury volume variations in the vessel. In fact, as mercury is an incompressible liquid, each decrease in Hg volume which can be recorded outside, of course involves the introduction of an equivalent Hg volume into the pores of the solid under test. The tendency is to obtain a continuous curve (today achievable only by extrapolation of points), correlating the values of volume and average radius of pores. In the practical application of this method, the above mentioned vessel is constitued by an ampoule, for instance a glass one, which is housed in an autoclave where a suitable liquid is introduced, partly also penetrating into the ampoule mouth. Once the autoclave is closed, said liquid is put under pressure with increasing values, to which negative variations in the ampoule mercury volume correspond, measured in correlation with the different pressure values.
The main problem faced up to now in the application of the described Drake method resulted to be the measurement of the above mentioned mercury volume variations in the ampoule. In the known apparatuses of this kind, it has been attempted to solve the problem by using a capillary of strictly constant diameter in correspondence with the ampoule neck and measuring the level variations of mercury in this capillary. In the known apparatuses, the latter measurement is carried out by connecting the ampoule bottom, by means of a metal probe crossing it, to an electric source, using a dielectric liquid for filling the autoclave and closing the electrical circuit by means of the mercury in the ampoule and a contact needle penetrating into the capillary down to the mercury level. The contact needle is mounted on a support which is made rotate in a threaded seat so that each turn of the support corresponds to one lead of the needle to which a certain mercury volume variation corresponds, measurable on the basis of the capillary diameter.
The analysis is carried out in subsequent steps, making the needle advance as far as the memory level and closure of the electrical circuit, then stopping the needle and increasing pressure until the circuit opens due to mercury meniscus lowering, interrupting pressure increase and making the needle advance again to repeat the cycle until pressure maximum values are reached.
From what reported herein, it is already possible to see which are the main disadvantages of the known apparatuses working as described above. First of all, measuring precision is strictly related to the precision achieved in the needle progress, and the instrument construction is very complex and expensive, involving numerous drawbacks in the use, especially due to the need of obtaining perfect sealing in correspondence to a movable component, such as the contact needle, capable of resisting to the very high pressures involved, which may reach values of 2000-2500 atmospheres. Secondly, considering that the volume variations in question are nonetheless minimum, measurement accuracy is far from being satisfactory. Further more, this accuracy is negatively affected by the possibility that mercury in the ampoule undergoes superficial oxidation, which modifies meniscus formed in such a way that, when the latter is taken away from the needle, the contact is not interrupted, this completely altering measurement result. Finally, measuring is carried out, as described, in a non-continuous system, allowing to obtain only a series of points which must then be extrapolated to obtain a Volume-Average Radius curve, which complicates and furtherly delays analysis already requiring very long time due to the described sequence of steps.